new start, from zero
use an extra stack to store the minimus numbers
binary search
optimize
still binary search
four cases:
case 1)
	num[mid -1] == num[mid] && num[mid] == num[mid + 1]
case 2)
	num[mid - 1] != num[mid] && num[mid] != num[mid + 1]
case 3)
	num[mid - 1] == num[mid] && num[mid] != num[mid + 1]
case 4)
	num[mid - 1] != num[mid] && num[mid] == num[mid + 1]
TLE edition
maybe STL is too slow
Do *not* use STL, too slow
linear search on a double-linked list works
fix bug in set()
when do set(), the key maybe already in the LRU list
just update the value
Merge branch 'fix_bug' into all_new

Conflicts:
	LRUCache.cpp
easy task
merge sort
using insert sort will get a TLE
use merge sort instead
wrong filename
easy task
so boring
be carefull about following case:
AAABBBCCC
easy task
easy task
easy task
easy task
hard to understand
easy task